Flux again, and smoke the mold again. I use a butane lighter to do it. I have a few that come out that way as well. If it wipes off with a cloth do not worry about it. Some of my best bullets have a frosty looking spot on the nose of them, in the same spot of the nose. As long as the base, and bands fill out properly I would not worry about it.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.